1. Field of the Invention
This invention directs itself to an electrostatic precipitation system wherein 100% particulate removal can practically be achieved. In particular, this invention directs itself to an electrostatic precipitation system having a laminar flow precipitator. To achieve laminar flow, the precipitator is divided into a charging section for imparting a charge to the particulates carried in a gas stream and a collecting section having a moving electrode disposed at a potential that is different from that of the charged particles, for attracting the charged particles thereto. More in particular, this invention pertains to a collecting section of a precipitator formed by a plurality of substantially parallel collecting passages, each passage being formed by a tubular member which is electrically coupled to a reference potential and in which a conductive fluid film coats an inner surface thereof and flows downwardly at substantially the same rate as the gas stream. Further, this invention directs itself to a laminar flow precipitator wherein the charging section and collecting section share a common reference potential electrode formed by a flowing fluid film, wherein the charging portion thereof is provided with a corona discharge and the collecting portion thereof is devoid of corona discharge.
2. Prior Art
The governmental requirements for preventing the emission of hazardous air pollutants is continually being made more stringent. Most prominent of the air pollutants being restricted, are toxic trace metals and their compounds. These compounds primarily exist in the form of particulate matter. Due to the nature of particulate formation in combustion processes, many of the trace metals, such as arsenic, cadmium, nickel, etc., as well as the high-boiling point organic hazardous air pollutants tend to concentrate on the fine, sub-micron sized particulates present in a flue gas. The problem of control of toxic trace metals and heavy organic pollutants therefore becomes largely a problem of fine particulate control. Other governmental regulations with respect to air emissions require control of sub-micron sized particles, as well.
Conventional collectors, electrostatic precipitators and fabric filters, are very capable of fine particulate control, but as the government requirements exceed 99.9%, they have difficulty in delivering consistent reliable performance, especially for the respirable particles in the 0.2 to 0.5 micron range. As the government regulations become more stringent, adequate control of toxic emissions will require particulate collection efficiencies of 99.95% or greater.
Conventional industrial electrostatic precipitators collect dry particulates in a parallel plate, horizontal flow, negative-polarity, single-stage system design. Collecting plate spacing generally ranges from 9 to 16 inches, and plate height can be up to 50 feet. Flow through the precipitator is always well into the turbulent range. Due to the turbulent flow, precipitator collection efficiency is predicted utilizing the Deutsch model, which assumes that the turbulence causes complete mixing of the particles in the turbulent core of the flow gas, and electrical forces are operative only across the laminar boundary layer. This model leads to an exponential equation relating collection efficiency to the product of the electrical migration velocity of the particles and the specific collecting area of the precipitator. The exponential nature of the equation means that increasing of the specific collecting area yields diminishing returns in the efficiency at the high collection efficiency levels. Therefore, the 100% collection efficiency level is approached only asymptotically in the turbulent flow case and cannot in actuality be reached, no matter how large the precipitator.
It has long been known that laminar flow precipitation provides many advantages over turbulent flow. In laminar flow, the flow stream lines are parallel and in the direction of flow; there is no force causing particles near the collecting surface to be thrown back into the central flow region. Therefore, the electrical forces tending to move the particles toward the collecting surface are effective across the entire flow cross-section, not just across the laminar sublayer. As a result, the equation which relates collection efficiency to the product of the electrical migration velocity of the particles and the specific collecting area defines a linear relationship, whereby collection efficiency is possible.
Besides the practical achievement of 100% collection efficiency, equivalent efficiencies in a laminar flow system can be achieved with a significantly smaller specific collecting area. The striking difference between the collection efficiencies of laminar flow, versus turbulent flow can be seen utilizing a typical utility fly ash emission system, calculating the specific collecting area (in square feet per thousand acfm) versus collection efficiency in two cases. In a turbulent flow system a specific collecting area of 230 is determined to be required at 99% collection efficiency, and is calculated to be over 800 at 99.99%. In a laminar flow calculation, on the other hand, the specific collecting area requirement is determined to range from 100 at 99% efficiency to only 160 at 99.99%. Thus, a turbulent flow precipitator is more than twice the size of an equivalent laminar flow precipitator at 99% collection efficiency and at 99.99% efficiency the turbulent flow precipitator must be more than five times larger than an equivalent laminar flow system. Although the advantages of laminar flow precipitation have been known, prior attempts to incorporate those principles into a working system have been unsuccessful or impractical for industrial scale applications. A major obstacle to achieving laminar flow in such systems has been the turbulence introduced by the corona discharge of the precipitator itself. When the charging section is separated from the collecting section, the holding force of the collecting section is reduced. However, the instant invention utilizes a substantially vertically and downwardly directed gas flow in combination with a two stage electrostatic precipitator design having separate charging and collecting sections with a moving electrode to achieve a practical laminar flow electrostatic precipitation system and collect the particulates from the gas stream, the moving electrode being formed by a conductive fluid flowing within each of a plurality of collection passages.
The best prior art known to the Applicants include U.S. Pat. Nos.: 1,329,844; 1,413,993; 1,944,523; 2,497,169; 2,648,394; 2,711,225; 3,495,379; 3,633,337; 3,830,039; 3,853,750; 4,072,477; 4,908,047; 5,009,677; 5,125,230; and, 5,254,155.
In some prior art systems, such as that shown in U.S. Pat. No. 5,254,155, an electrostatic precipitator system is disclosed wherein a single-stage structure is provided. Such systems provide a plurality of passageways that are defined by a honeycomb structure for gas flow upwardly therethrough. Stationary rods extend into each passageway, the rods being coupled to the negative output of a power supply, while the walls of the honeycomb passageways are coupled to a reference potential. Removal of the collected particulates is accomplished by washing them downwardly utilizing a liquid mist (water) collected from the gas stream. The liquid mist is introduced into the gas flow upstream of the electrostatic precipitator electrodes, and is introduced solely for cleaning contaminants from the collecting electrodes. Since a corona discharge is maintained throughout the length of the honeycomb passages, laminar gas flow is not achieved. Further, since the water flows in a direction opposite to that of the gas stream, there cannot be a net zero velocity between their respective flow rates.
In other systems, such as that disclosed by U.S. Pat. No. 2,648,394, the gas to be cleaned flows downwardly through a housing in order to be directed upwardly through the precipitator, which is defined by a plurality of tubular members having centrally disposed electrodes extending axially therethrough. Here again, a single-stage system is provided wherein laminar flow of the gas is not achieved. Spray nozzles are also provided for introducing water droplets into the gas inlet conduits which serve to flush deposited material out of the tubular members. Again, the water flow is opposite that of the gas flow and thus cannot contribute to producing a laminar flow of the gas.
In other systems, like those shown in U.S. Pat. Nos. 5,009,677 and 2,497,169, single-stage electrostatic precipitators are formed utilizing a plurality of vertically oriented tubular collecting electrodes through which a discharge electrode extends axially therethrough, for establishing a corona discharge throughout the length of the tubular electrode.
None of these prior art systems direct themselves to achieving laminar flow of the particulate-laden gas. Additionally, these prior art systems do not direct the gas downwardly through electrostatic tubular collecting electrodes which are devoid of corona discharge. Further, none of these prior art systems disclose or suggest the use of a conductive fluid film as a moving collection electrode to attract and carry away particulates while simultaneously contributing to the establishment of laminar flow of the gas, and thereby result in a less efficient system than that provided by the instant invention.